1. Field of the Invention
The present invention relates to a color picture tube device used in televisions and the like, and in particular relates to techniques of correcting raster distortion.
2. Related Art
One type of raster distortion is called inner distortion. Inner distortion includes upper and lower inner pincushion distortion and upper and lower inner barrel distortion. The upper and lower inner pincushion distortion refers to a situation where the vertical amplitude of the electron beams inside the raster becomes insufficient in a direction toward the horizontal center of the screen. The upper and lower inner barrel distortion refers to a situation where the vertical amplitude of the electron beams inside the raster becomes excessive in the direction toward the horizontal center of the screen.
Such inner distortion can be effectively corrected by providing a means of generating a correction magnetic field in a region where deflection magnetic fields are generated by a deflection yoke. For example, a technique of placing a pair of upper and lower permanent magnets in the gaps between the horizontal deflection coil and the picture tube is known to remedy the upper and lower inner barrel distortion (Published Unexamined Japanese Patent Application No. H06-283115).
However, permanent magnets have relatively wide variations in the amount of magnetization, due to manufacturing reasons. Therefore, even if the pair of upper and lower permanent magnets are provided, there is a possibility that they may deviate from a magnetic field intensity tolerance set at the time of designing the picture tube device. Since the pair of upper and lower permanent magnets are situated near an area where electron beams pass through, such variations in magnetic force acutely affect convergence. If the pair of upper and lower permanent magnets deviate from the magnetic field intensity tolerance, misconvergence occurs which constitutes a significant problem for the use of the picture tube device.
This problem may be solved by employing coils that can deliver a desired magnetic field intensity more easily than permanent magnets. In general, however, a coil that delivers the same level of magnetic field intensity as a permanent magnet is larger in size than the permanent magnet. Accordingly, such a coil cannot be placed in a limited space between the horizontal deflection coil and the picture tube.